The following description includes information that may be useful in understanding the present invention. It is not an admission that any of the information provided herein is prior art or relevant to the presently claimed invention, or that any publication specifically or implicitly referenced is prior art.
Atomic clocks, which rely on characteristic electronic energy transitions to generate a frequency standard, provide highly accurate measurements of time intervals that are integral to a number of economically important devices and systems, notably global positioning systems. Synchronized networks of such clocks are used to generate the International Atomic Time standard, which serves as the basis of the Coordinated Universal Time used for conventional timekeeping throughout the world.
A typical atomic clock utilizes a controllable electronic oscillator that generates a microwave frequency corresponding to a hyperfine energy transition that is observed in a cesium or rubidium atom. Absorption of the emitted microwave by an atomic reference cell (for example, a vapor cell) containing an appropriate atom or molecule provides feedback for a control loop that adjusts the electronic oscillator to fix the emitted frequency. While relatively simple conceptually, in practice correction for the initial stabilization of the electronic oscillator, frequencies generated by other electronic transitions, and environmental factors such as temperature changes greatly complicates the design and operation of such atomic clocks.
More recently, atomic clocks based on optical frequencies have been developed. For example, U.S. Patent Application Publication No. 2013/0003766 (to Savchenkov et al) describes an optical atomic clock in which laser output is coupled to both a tunable optical resonator and an atomic reference. All publications identified herein are incorporated by reference to the same extent as if each individual publication or patent application were specifically and individually indicated to be incorporated by reference. Where a definition or use of a term in an incorporated reference is inconsistent or contrary to the definition of that term provided herein, the definition of that term provided herein applies and the definition of that term in the reference does not apply. In such an approach, absorption of the laser output by the atomic reference forms part of a feedback loop used to stabilize the tunable optical resonator. The stabilized optical resonator provides, via nonlinear wave mixing of the laser output, an optical comb that is stabilized relative to the atomic reference. This stabilized optical comb is then used to generate an RF signal. In such a design, however, the wavelength used is necessarily limited to one that is compatible with both the optical resonator and the atomic reference.
Thus, there is still a need for an optical atomic clock that decouples optical resonator stabilization from optical comb generation.